Magnetosphere-Ionosphere Coupling and Aurora at Jupiter and Saturn Emma Bunce and Stan Cowley University of Leicester, UK Image credits: John Clarke/Boston University /NASA HST

Magnetosphere-ionosphere coupling and field-aligned currents in planetary systems The field-aligned currents that flow between the ionosphere and magnetosphere in planetary plasma systems are fundamental to the processes that transfer stress between these regions via the planetary magnetic field (see, e.g., the review by Cowley [2000]). At Earth, the momentum is directed from the solar wind/magnetosphere to the ionosphere, while at Jupiter and Saturn it mostly goes the other way… Taken from Cowley [2001] Taken from Cowley et al [2004]Taken from Cowley et al [2003] Ionospheric Flows and Currents

Initial theoretical considerations focused on currents associated with plasma sub-corotation, taken from Cowley, Bunce & O’Rourke [2004] - plasma angular velocity starts low on open field lines, increases sharply on closed field lines across the OCB, and near-rigidly corotates at lower latitudes (based on Voyager) - ionospheric meridional current is positive equatorward, assuming a fixed effective Pedersen conductivity Σ P * of 1 mho - field-aligned current density just above ionosphere, determined by the gradient of the meridional current, downward over polar region, upward at OCB (model auroral oval), and weaker upward again at low latitudes - “lagging” azimuthal field just above ionosphere associated with the current system, varying along field lines according to ρB φ ≈ constant (ρ is perpendicular distance from axis) Observationally, we work from the bottom up! - we observe the “lagging” B φ field in the magnetosphere - we infer the ionospheric meridional current profile & hence field-aligned currents - using observed angular velocities we also infer the (non- constant) Σ P * Plasma angular velocity Ionospheric meridional current Field-aligned current density “Lagging” azimuthal field above ionosphere ω/ΩSω/ΩS I’ hP /MA rad -1 j || /nA m -2 B ϕ /nT Southern hemisphere Northern hemisphere To equator up down What have we learned from Cassini about M-I coupling currents at Saturn?

Cassini gave us the first signatures of field-aligned currents in Saturn’s magnetosphere HST observations with high-latitude in situ Cassini observations See Bunce et al., 2008a; Cowley et al., 2008; Talboys et al., 2009a,b & 2011 Image A : UT Image B Image B : UT Magnetically mapped spacecraft footprint Field-aligned current signatures consistently seen on high-latitude orbits in MAG data NIGHTSIDE field-aligned current signatures seemed to show two distinct morphologies (above). They lie consistently on closed field lines, equatorward of polar cap boundary DAYSIDE observation in 2007 by Cassini MAG/CAPS-ELS show a large-scale field-aligned current present near the open- closed field line (or polar cap) boundary AB B ϕ /nT

Plots show results derived from ~30 post-midnight high-latitude Cassini passes across southern auroral zone in (a) Meridional current profiles: red theoretical profile based on Cassini ω/Ω S data (b) and a fixed conductivity of 0.75 mho, FAC directed down over polar region, sharp upward at OCB where ω/Ω S sharply increases, and weak upward again at large co-latitudes (similar to CBO model) -Data shows distributed downward current over the polar region as predicted, but continuing more strongly downward on outer closed field lines -(c) Due to enhanced conductivity derived in bottom panel (red line) -Sharp upward (auroral) current is centred ~2° equatorward of OCB as the conductivity drops -Lower latitude upward currents appear not to exist despite continuing plasma subcorotation, due to low ionospheric conductivity Results show that while the basic theoretical ideas have survived, variations in ionospheric conductivity are at least as important as plasma angular velocity in determining the overall field- aligned current profiles See Hunt et al [2014] Statistical studies of Saturn’s SH field-aligned currents ω/ΩSω/ΩS I M /MA rad -1 Σ * p /mho Black-Wilson et al. [2009] Green-Muller et al. [2010] Blue-Carbary & Mitchell [2014] * Thomsen et al [2014] Red = theory Dots = data Solid = data Assumed 0.75 mho

Plasma angular velocity/Ω J Ionospheric meridional current/MA FAC density/nA m -2 “Lagging” azimuthal field above ionosphere/nT For the equivalent Jupiter sub-corotation model proposed by Cowley et al [2005], shown left, we are conversely more confident in the description of the lower-latitude upward current associated with corotation breakdown at ~20 R J due to outward transport of Io plasma, first appropriately modelled by Cowley & Bunce [2001] - the model provides a good description of the location, width, and intensity of the main auroral oval, and the energy (~ keV) of the electron primaries determined by UV spectroscopy The description of the polar region is essentially speculative - how much open flux is in the system, and where is it located? -what is the origin of the “swirl” auroras, and to what magnetospheric regime do they map ? See Vogt et al. [2011,2015] for auroral mapping discussion HST image Grodent et al [2003] Main oval ω/ΩSω/ΩS I hP /MA j || /nA m -2 B ϕ /nT

View model results for middle magnetosphere field sweep-back due to plasma sub-corotation Black lines show field lines in the magnetic meridian mapping from co-lats of 5  -25  in the ionosphere - black dotted box is the equatorial current disk Green lines (also field lines) show the regions of upward-directed FAC at the OCB and in the MM, the latter closing in the equatorial current sheet Red and blue lines show contour maps of B  produced by the FAC system, -ve in NH and +ve in SH, from  2 nT to  50 nT - overall a ‘lagging’ field configuration is produced in the MM by the plasma sub-corotation - angular field deflections are a ~5  outside the MM current sheet, reducing to a few tenths of a degree closer to the magnetic axis Open-closed boundary FAC Main oval FAC -5 nT -10 nT -20 nT +10 nT +5 nT +20 nT See Cowley et al. [2008]

We can project the Juno orbits onto a magnetic meridian – at start, middle, & end of mission as examples - trajectory oscillates at planetary period of 9.9 h due to rotation of magnetic axis around the spin axis - blue dots plotted every 10 hours relative to the periapsis point - orbit traverses polar magnetosphere at low altitudes both north & south throughout the mission - at start of mission the line of apsides is initially near the equatorial plane - due to non-spherical Jupiter, apoapsis rotates south, line of apsides at ~35  at end of mission - thus also traverses wide regions of high-lat magnetosphere previously unexplored Start of missionMiddle of missionEnd of mission Outbound Inbound

Primary observable of MI coupling current system is azimuthal field B  Middle panel shows model B  versus time for  50 hr either side of periapsis of the Juno orbit (central blue dot on trajectory) Lower panel expands results for north polar pass from 1.2 to 0.2 hr prior to periapsis - time scale for passes over main oval FACs is ~2 min - time scale for passes over polar FACs is ~10 sec Middle of mission Inbound Outbound Polar arc FACs Main oval FACs B ϕ /nT Show a couple of results for the ‘middle of mission’ case by way of illustration

See Hunt et al [2014] A major unanticipated effect at Saturn is the “planetary period oscillation” (PPO) currents which strongly modulate the field-aligned currents appear to be driven by rotating twin-vortex flows in the thermosphere of uncertain origin, see e.g. Jia and Kivelson [2012] separate systems driven from the N and S hemispheres with slightly differing seasonally- varying periods, as sketched below for the SH system See Southwood & Cowley [2014] Panels (b) and (d) on right show SH cases where the PPO phase is such that the current is small, so the sub-corotation system dominates Panels (a) and (c) show PPO phases near ~90° and ~270° where comparable downward and upward PPO currents are superposed on the equatorward part of the subcorotation profile, reducing the overall current to near zero in the first case and near doubling it in the second The most recent results show that 2008 NH data are yet more complicated since the effect of NH and SH currents are both present at comparable amplitudes, thus providing the first direct evidence of inter- hemispheric PPO current flow as proposed by Southwood & Kivelson [2007] No evidence to date of any corresponding effect at Jupiter, but there will certainly be some surprises ! a b cd

Nichols et al. [2008] have shown that the auroral oval oscillates with the SKR period (SH) Similarly, Badman et al. [2012] have shown that the intensity of the infrared aurora is modulated by the “magnetosphere oscillation phase” Bunce et al [2014] showed evidence for auroral oval motion with northern hemisphere phase, although Hunt et al. [submitted 2015] show the northern hemisphere behaviour is more complex. Do we see planetary period oscillation phase modulation of the auroral oval? Hunt et al. [2014] have shown that the overall high- latitude field-aligned currents in the SH are structured into 4 sheets – from the pole towards the equator – down/up/down/up Thus, we may expect to see auroral arcs associated with this 4 sheet effect. Some evidence of this exists in the UV data.

What about the solar wind interaction? The main well-documented effect at Saturn is the “auroral storms” produced by strong compressions of the magnetosphere by CIRs/CMEs in the solar wind - the usual near-circular auroral oval becomes brightened on the dawn side and expands strongly towards the pole, see Clarke et al. [2005] - suggested by Cowley et al [2005] that this is due to induced bursts of nightside reconnection that close a significant proportion of the open tail flux, similar to substorms at Earth - corresponding in situ tail data at the expected time of a CIR compression is shown below, with a SKR burst, hot plasma injection, and field dipolarization - occurrence statistics indicate one event every ~6 days, each lasting ~16 h See Meredith et al [2014] See Bunce et al [2005] HST images

Storm 11 Storm 1 No storm yet observed from start to finish, but from the overall ensemble the suggested time sequence is - Storm 12 Storm 3 ~1-3 h ~1-3 h onset post-midnight tail reconnection, expands rapidly dawnward (~2xcorot), more slowly poleward forming hot plasma bulge ~3-8 h ~3-8 h bulge expands eastward (~0.6xcorot) to fill whole of dawn sector Storm 2 Storm 7 ~8-12 h ~8-12 h auroras bifurcate with active poleward edge & structured bulge, which vents into noon and dusk sector ~12-16 h ~12-16 h activity at poleward edge dies away from nightside towards pre-noon, storm auroras dissipate, usual dawn arc reforms, injected plasma reaches midnight sector Storm 10 Storm See Prangé et al. [2004], Badman et al. [2005]; Meredith et al [2014]; Nichols et al. [2014]

Lamy et al We interpret the highly unusual field-aligned current signatures (seen at the same time as the s/c entered the SKR source region) as being the in situ consequence of a compression of the magnetosphere – inducing rapid tail reconnection [like that suggested by Cowley et al., 2005], enhanced auroras and radio emission [agrees with Lamy et al., 2010] Cowley et al., 2005 What does the in situ data at high-latitudes look like during these storms? - Unusual events excluded from FAC statistical studies hold the clues! SKR source region Enhanced field-aligned currents Plus dayside field-aligned currents and hot plasma at *very* high latitude See Lamy et al., 2010 See Bunce et al., 2010

If compression events close tail flux, dayside reconnection must open it at low rates (tens kV) over many-day intervals [Jackman et al, 2004] - Evidence in dayside auroras for bursty (FTE- style) reconnection signatures in post-noon hours (possibly supressed pre-noon by large flow shear across boundary) See Radioti et al [2013], also Badman et al. [2013] Cassini UVIS images See Meredith et al [2013]HST images Also evidence found in HST images when Cassini was in the upstream solar wind/IMF that such patchy variable post- noon auroras are present when IMF B z is positive (left images for visit H3), and not when it is negative (right images for visit I7) We thus have evidence that the Dungey cycle is active at Saturn, though in modified form and on longer time scales than at Earth It remains to be seen whether there are any corresponding effects at Jupiter - we know the aurora brighten in response to CIR/CME- related events, but exactly where and when remains to be determined [see Gurnett et al., 2002; Nichols et al. 2009] IMF B z +ve IMF B z -ve

Summary: Cassini has already taught us a great deal about Saturn’s polar magnetosphere, field- aligned current systems, and aurora. It is much more complex than we anticipated. The final proximal orbits will bring new insights into the auroral regions and allow a full study of the variation of the field-aligned currents systems with solar rotation, planetary rotation, solar cycle and Saturn’s seasons! Juno will provide the first full study of Jupiter’s polar magnetosphere and auroral regions from (very!) close proximity: we look forward to comparing our model predictions with the first in situ data. How do Jupiter’s auroras vary with planetary rotation? What is the nature of the solar wind interaction? We anticipate some surprises!